1. Field of the Invention
The invention relates to reading of magnetic memory and more particularly, to reliably reading magnetic Random Access Memory (RAM) such as spin-transfer torque magnetic random access memory (STTMRAM).
2. Description of the Prior Art
Magnetic random access memory (MRAM), such as spin transfer torque magnetic random access memory (STTMRAM) is one of the next generations of non-volatile memory currently under development. During programming of the MRAMs, the magnetic junction of the MRAM is programmed to a ‘0’ or ‘1’ state by changing the resistance thereof, using current to effectuate the change in resistance. The data of the magnetic junction—magnetic tunnel junction (MTJ)—is considered to be ‘0’ when the resistance of the MTJ is low when the orientation of the two magnets of the MTJ are aligned, and ‘1’ when the magnetic orientation of the two junctions are in opposite or near opposite alignment. Since the resistance of the MTJ is inversely proportional to the MTJ area, in a large memory, the MTJ resistance varies across the die onto which the MRAM is formed and follows a Gaussian distribution. In a large memory, the low resistance of the MTJ has an average value with a certain variation around it specified with a certain sigma. The programmed MTJs have higher value of average resistance and also higher sigma because programming causes more variations in the resistance. To read the MTJ, data need be extracted and in this regard, a comparison is made between the resistance value of the MTJ being read to a reference MTJ that has the resistance value of:Rref=(Rlavg+Rhavg)/2  Eq. (1)
“Rref” represents the resistance of a reference MTJ, which is derived by calculating the average of the “Rlavg” and “Rhavg”. “Rlavg” represents the average value of the distribution of “Rlow” and “Rhavg” is the average value of the distribution of “Rhigh”. “Rlow” represents the resistance of the MTJ at a ‘0’ or low state, whereas, “Rhigh” represents the resistance of the MTJ at a ‘1’ or high state. For a better understanding, FIG. 1, well known in the art, is presented.
FIG. 1 shows a prior art graph of the resistance distribution of MTJs. The x-axis shows resistance while the y-axis shows the number of MTJs. Two Gaussian distributions are shown, one is the Rlow distribution and another is the Rhigh distribution. A reference resistance, “Rref”, is shown to be in the central middle between Rlavg and Rhavg. The Rlow and Rhigh distributions are shown to be non-overlapping in FIG. 1 in that Rref is comfortably distant from these distributions allowing for reliable assessment of the value of Rlavg and Rhavg without concern for one being mistaken for the other.
However, occasionally, the Rlow and Rhigh distributions are not exactly reflected in FIG. 1 and in fact, some time they overlap, leading to erroneous sensing or reading of MTJs. Sensing presents a challenge when Rlow and Rhigh distributions are close or overlap. Thus, there is a need for a reliable sensing scheme for MRAMs when the R distributions are close or overlap.